Autofocus calibration for long lenses

ABSTRACT

A method for calibrating a focus point for a camera lens may include capturing a reflection of a focus point measuring device that is affixed to the camera. The method may include evaluating a captured image of the reflection to measure a calibration amount for a focus point, and adjusting a focus point of a lens of the camera by the calibration amount. The focus point measuring device may include a substantially planar target surface defining a plane, and a ruled target surface inclined at substantially 45° to the substantially planar target and extending through the plane thereof, marked to indicate respective distances in front of and behind the plane. The device may further include a fixture for holding the substantially planar target surface and the ruled target surface in a defined orientation to the camera, enabling performance of the method.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority pursuant to 35 U.S.C. §119(e) to U.S.provisional application Ser. No. 61/428,015, filed Dec. 29, 2010, whichis hereby incorporated by reference, in its entirety.

BACKGROUND

1. Field

This application relates to methods and systems for calibrating a focuspoint for a camera lens.

2. Description of Related Art

High-end cameras and lenses are often susceptible to back-focusing orfront focusing problems, which may be described as automaticallyfocusing on a point that is slightly behind or in front of the pointthat the photographer intends to focus on, usually the surface of someobject. These problems arise from manufacturing tolerances used inmass-production of camera lenses, coupled with the fact that,particularly for long lenses, any imprecision in the manufacture of thelens or its auto focus system may show up as a noticeable lack of focus,especially when automatically focusing on a distant object. Imprecisioncan also build up in unpredictable ways from coupling a lens to otherelements in the optical path, such as a camera body.

To compensate for unavoidable imprecision in manufacturing, many cameramanufacturers build cameras equipped with Micro-AF adjustmentcapability. Such capability enables the user to instruct the camera toautomatically apply a degree of plus or minus focus correction inresponse to being coupled to a defined lens. It remains up to the userto determine an amount of focus correction, sometimes called acalibration amount, that should be applied for each particular cameraand lens combination.

Various methods may be used to determine an amount of focus correctionfor a particular camera/lens combination. For example, “LensAlign”™,available from www.whibalhost.com, is a device that may be used tomeasure an amount of back focus or front focus, and thereby determine anamount of focus correction to apply. A LensAlign device is comprised ofa base for holding the camera which can sit flat on a table, or bemounted on a light stand or tripod, together with a front standard and arear standard. The front standard provides a focus target and is markedwith precision rulings similar to a measurement ruler or the like. Therear standard is marked with several bulls-eye targets that line up withholes on the front standard, for aligning the camera with the frontstandard so that the camera and focus target are exactly plane parallel.Alignment is performed by sighting through the camera and lens until thebulls-eye targets are exactly aligned with holes in the front standard,then using the camera's autofocus to focus on the front standard,capturing an image and evaluating the image. Because of the precisionrule markings on the front standard, the amount of front or back focusis readily visible from the captured image. Further details are shown athttp://www.whibalhost.com/lensalign/how-lensalign-works.html.

SUMMARY

Methods and systems for calibrating a focus point for a camera lens aredisclosed, including methods and systems using a fixture and alignmentdevice attached to a camera.

In an aspect, a method for calibrating a focus point for a camera lensmay include, using a camera, capturing a reflection of a focus pointmeasuring device, wherein the focus point measuring device is affixed tothe camera. The method may further include evaluating a captured imageof the reflection to measure a calibration amount for a focus point. Themethod may further include adjusting a focus point of a lens of thecamera by the calibration amount. The method may include selecting thefocus point measuring device to be in a particular form, as described inmore detail herein.

In related aspects, the method may include fastening the fixture to thecamera using a removable fastener. For example, the method may includeinterposing the fixture between a tripod and the camera for mounting thecamera to the tripod, and attaching the tripod to the fixture. In thealternative, the method may include attaching the fixture to a hot shoefor the camera. In another alternative, the method may include attachingthe fixture to the lens.

In other aspects, the method may include holding the substantiallyplanar target surface so that a plane thereof is perpendicular to areflected leg of a line of sight between the mirror and a substantiallyplanar target. In the alternative, or in addition, the method mayinclude holding the substantially planar target surface so that anon-reflected leg of the line of sight between the lens and the mirroris not parallel to the second portion of the line of sight.

In an aspect of the method, or separately from practice of the method,the focus point measuring device may be provided in a particular form.In an aspect, the focus point measuring device may be provided as anapparatus including a substantially planar target surface defining aplane, and a ruled target surface inclined at substantially 45° to thesubstantially planar target and extending through the plane thereof,marked to indicate respective distances in front of and behind theplane. The apparatus may further include a fixture for holding thesubstantially planar target surface and the ruled target surface in adefined orientation to a camera, whereby a reflection of thesubstantially planar target surface and the ruled target surface is madevisible through the camera and its lens pointing at a mirror along aline of sight, in which reflection the substantially planar targetsurface and the ruled target surface are centrally disposed in a fieldof view along the line of sight and the substantially planar targetsurface is substantially perpendicular to the line of sight.

In related aspects, the apparatus may further include a removablefastener for fastening the fixture to the camera. The fixture may beconfigured to mount to the camera in various different ways. In anaspect, the fixture may be configured to mount interposed between atripod and the camera, for mounting the camera to the tripod. In suchconfigurations, the apparatus may further include the tripod attached tothe fixture. In an alternative aspect, the fixture may be configured toattach to a hot shoe for the camera, for example by including a malemounting piece shaped to insert in a hot show female mounting bracket.In another alternative, the fixture may be configured to attach to thelens, for example using a circular (e.g., annular) clamp.

In another aspect, the fixture may be configured to hold thesubstantially planar target surface so that the plane thereof isperpendicular to a reflected leg of the line of sight between the mirrorand the substantially planar target. For example, the fixture may beattached to the substantially planar target surface and include a cameramounting interface that is oriented to align the target surface within afew degrees of perpendicular to the camera line of sight. The degree ofinclination may be fixed or adjustable.

In another aspect, the fixture may be configured to hold thesubstantially planar target surface so that a non-reflected leg of theline of sight between the lens and the mirror is not parallel to thesecond portion (i.e., the reflected leg) of the line of sight. Forexample, the fixture may be attached to the substantially planar targetsurface and include a camera mounting interface that is oriented toalign the target surface at a non-zero inclination of perpendicular tothe camera line of sight; for example, between 5 and 10 degrees.

In another aspect, the substantially planar target surface defining aplane and the ruled target surface are formed in a molded polymermaterial. In addition, may formed in a molded polymer material, eitheras a separate piece or molded in the same piece as the substantiallyplanar target surface and the ruled target surface.

Further embodiments, aspects and details of the method and apparatus forcalibrating a focus point for a camera lens are presented in thedetailed description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The present technology, in accordance with one or more variousembodiments, is described in detail with reference to the followingfigures. The drawings are provided for purposes of illustration only andmerely depict illustrative examples.

FIG. 1 is an isometric view of a focus point measuring deviceillustrating aspects of the present technology.

FIG. 2 is a system diagram illustrating aspects of using a focus pointmeasuring device with a camera and other elements to calibrate a focuspoint for a camera lens.

FIG. 3 is a flow chart showing examples of operations in a method forcalibrating a focus point for a camera lens.

DETAILED DESCRIPTION

Notwithstanding the advantages of LensAlign or similar devices andmethods, they may be subject to certain disadvantages. One disadvantagemay relate to the amount of distance required to set up the measuringdevice and capture an image. For example, using a multiplier of 50 timesthe focal length as recommended by one prominent lens manufacture, alens with a focal length of 400 mm requires a 20 meter calibrationdistance. Many photographers may not have convenient access to asuitable calibration space at least 20 meters long. In addition, theprocess of aligning the calibration device to the camera may beunnecessarily time consuming or may unnecessarily introduce apossibility for error.

The present disclosure describes a method and apparatus that overcomesthese and other limitations of the prior art, by providing aprecision-ruled focus target in a fixture attached to the camera body.For example, the camera may be mounted on a focus target which in turnmounts on a tripod. In the alternative, the tripod may be provided withthe focus target built into it or attached below, beside, or above thecamera mount. In another alternative, the focus target mounts on thehot-shoe of the camera. A less preferred alternative is for the focustarget to be mounted on the lens itself, such as in the threads. Thismay introduce the disadvantage of having to use a focus point other thanthe center point, as the center point would be the lens itself.

To use the alignment fixture, the camera is pointed at a flat mirrorwhich reflects the focus target. The user focuses on the reflection ofthe target appearing in the mirror, and captures an image of thereflection. This effectively doubles the distance to the focus target,without having to find additional linear space. For example, a 400 mmlong lens can be effectively calibrated in a distance of 10 m, using thefixture with a mirror, instead of the 20 m that would be required usinga prior art calibration method. In addition, there is no need tomanually align the focus target to the camera, because the fixture isconfigured to hold the focus target in a defined alignment with thefocal plane of the lens. When the camera is in the center of the frameas seen in the reflection, the focus target is necessarily flat relativeto the focal plane of the camera. Once having captured an image of thefocus target, the user may determine the calibration amount in aconventional manner, by examining the image of the focus target to seewhere on the ruled target the point of focus actually lies.

In a one implementation, the mirror is angled very slightly up, and thetarget is angled very slightly down so that the camera is able to centeron the focal target as flat relative to the focal plane of the camera.This angling may be used to compensate for the fact that an opaque focustarget cannot be placed directly in front of the camera lens withoutobstructing the field of view. Instead of “angling up” and “anglingdown,” in any other similar complimentary configuration may be used. Inthe alternative, the focus target may transmit light in one directionbut not another, as a one-way mirror.

Referring to FIG. 1, an example of an apparatus 100 for measuring afocus calibration amount is shown in perspective view. The example issimplified to illustrate general aspects of a suitable measurementapparatus, and should not be understood as representing a detaileddesign for such an apparatus with respect to aspects that are notdiscussed. The apparatus 100 may comprise a substantially planar andopaque target surface 102 defining a focus plane. The target surface maybe marked or imprinted with one or more markings to aid in centering andfocusing thereon using a camera's autofocus mechanism. The targetsurface should be large enough in area and configured with surfaceproperties to reliably reflect the camera's sounding signal, for examplean infrared pulse.

As used herein, “substantially planar” means planar within reasonablemanufacturing tolerances, so that the surface is apparently planar sofar as is perceptible using the focusing optics of the camera lens. Theterm “planar” means in the form of a plane; this may include but is notstrictly limited to a flat surface target 102 as depicted in FIG. 1. Onepurpose of the substantially planar target surface is to provide anobject that can be brought into sharp focus by aligning a focus plane ofthe camera lens as closely as possible with the target. Therefore, ifthe focal plane of the camera lens is not substantially flat (i.e., itis curved in some detectable amount), then the target surface may becurved to parallel and curvature in the lens' focal plane. Anotherpurpose for the target surface 102 may be to reflect the camera'ssounding signal for autofocus back to the camera. For example, a flat orslightly parabolic target surface may facilitate this purpose.

The apparatus 100 may further comprise a ruled target surface 104inclined at substantially 45° (for example, 45°±5°) to the substantiallyplanar target 102 and extending through the plane thereof. The relativeposition and orientation of the planar surface 102 and ruled surface 104should be fixed by configuration of the supporting fixture 108.

The ruled surface 104 is marked to indicate respective distances infront of and behind the plane of the planar surface 102. For example,the ruled surface may be marked zero at the point of intersection withthe planar surface 102, with positive and negative markings onrespective sides of the zero point, as shown. A more detailed example ofa ruled marking surface is provided by the LensAlign™ device referencedabove, which has staggered rule markings resembling a Vernier scaleexhibiting a fine regular pattern for more easily ascertaining the exactfocus point. The ruled surface 104 is not intended to reflect thecamera's sounding signal for autofocus, and as such may be positionedimmediately adjacent to or near the surface 102 so as to appear inphotographs taken of the planar surface 102 without interfering withdistance sounding by the camera's autofocus mechanism.

The apparatus 100 further comprises a fixture 108 of any suitablestructural material configured for holding the substantially planartarget surface 102 and the ruled target surface 104 in a definedorientation relative to a camera. For example, a camera body may beattached using a fastener to a receptacle 106 (e.g., a threaded hole),bearing against a mounting surface adjacent to the receptacle 106. Thefixture holds the two surfaces 102, 104 at a relative orientation to anattached camera such that a reflection of the substantially planartarget surface and the ruled target surface is made visible through thecamera and its lens pointing at a flat mirror along a line of sight. Insuch a reflection, the substantially planar target surface 102 and theruled target surface 104 are centrally disposed in a field of view alongthe line of sight and the substantially planar target surface 102 issubstantially perpendicular to the line of sight.

Apparatus 100 lacks any feature for optically aligning the planarsurface 102 to a camera lens. Such a feature is not necessary, becausethe fixture 108 includes a mechanical alignment feature, for example oneor more mounting surfaces (for example three mounting pads, not shown)to align the camera body and hence its lens with respect to the targetsurface 102.

FIG. 2 shows the apparatus 100 attached to a support structure, forexample a tripod 212 and to a camera 202. In the illustrated embodiment,the apparatus 100 is interposed between the camera body 206 and thetripod 212. In the alternative, the apparatus 100 may be fixed above thecamera body 206, for example by mounting to the hot shoe 208, with thecamera body 206 mounted directly to the tripod 212. Either way, whenmounted to the apparatus 100, the camera 202 and its lens 204 are heldrelative to the apparatus 100 using any suitable mounting attachment atan orientation whereby a reflection of the substantially planar targetsurface 102 and the ruled target surface 104 is made visible through thecamera 202 and its lens 204 pointing at a flat mirror 216 along a lineof sight 210. In such a reflection visible in the mirror 216, thesubstantially planar target surface 102 and the ruled target surface 104are centrally disposed in a field of view along the line of sight 210and the substantially planar target surface 102 is substantiallyperpendicular to the line of sight 210, for example, at an angle of90°±5°.

Typically the mirror 216 may be attached to or supported by a wall orother support structure (not shown) that is substantially perpendicularto a floor or other support surface. Meanwhile, the base of the tripod212 may also rest on the same floor or other support surface, and beleveled so that when a camera is attached to the apparatus 100 ordirectly to the tripod 212, line of sight 210 for the lens 204 isparallel to the floor and thus perpendicular, or nearly perpendicular,to the reflective surface of the mirror 216. Thus, when the lens ispointed at the mirror until the planar surface 102 is in the center ofits field of view, the planar surface 102 will be parallel, or nearlyparallel, to the reflective surface of the mirror 216. The foregoingexample demonstrates how the mechanical system 200 comprising a mirrorsupport (e.g., wall), floor, tripod, and apparatus 100 can be used toquickly align the working surfaces of the apparatus 100 with the centralline of sight 210 for the lens 204 and camera body 206, without using anoptical alignment device. Other methods of mechanical alignment may alsobe used, characterized by the disclosed feature of mechanicallyattaching the apparatus 100 to the camera 202.

In the depicted embodiment, the planar target 102 is slightly off centerof the center of the lens 204, which is naturally necessary to avoidobstructing the line of sight 210, at least when the planar surface isconstructed of an opaque material. Therefore, the line of sight 210comprises a first leg 214 between the lens 204 and mirror 216 inclinedat an angle “α” to a second leg 216 between the mirror 216 and planarsurface 102. The angle α may be fairly small, and may be determined bythe arctangent of the perpendicular distance between the center of theplanar surface and the first leg 214 line of sight divided by the lengthof the leg 214 (i.e., one-half the total line of sight 210 length). Forexample, assuming a 10 cm perpendicular offset of leg 214 from thecenter of the target 102 and a total line of sight length of 20 m, theangle α equals the arctangent of 0.1 m/10 m, which is approximately0.57°. The fixture 108 may be configured to hold the camera 202 so thatthe angle α may be fixed or adjusted as needed for different camerabodies and alignment lengths. For example, once the system 200 is inplace the user may turn a fine adjustment screw to pivot the camera 202slightly up or down (or side to side) until the center target of theplanar surface appears in the center of the camera's field of view.

Consistent with the foregoing, a user may use the focus measurementapparatus 100 or equivalent device to determine an amount of focusadjustment for an autofocus feature of a camera/lens combination, usinga method 300 as shown in FIG. 3. Initially, the user configures theapparatus 100 with other elements of a system 200 as shown and describedabove. Then, using a camera, a user may capture 302 a reflection of thefocus point measuring device (e.g., apparatus 100). Then, after taking aphotograph of the reflection, the user evaluates 304 a captured image ofthe reflection to measure a calibration amount for a focus point. Theuser may then adjust 306 a focus point of a lens of the camera by thecalibration amount.

By way of example, a distinguishing feature of this method 300 is thatthe focus point measuring device is affixed to the camera instead ofbeing a free standing object. Another point of distinction is that areflection of the measuring apparatus is captured, for example byphotographing the device as reflected in a mirror, instead of anunreflected image of the measuring device. Yet another point ofdistinction is that no optical alignment device is needed for aligningthe measurement device to the camera; instead a mechanical fixture thataffixes the camera to the measuring device is used to align the twobodies. The appended examples further illustrate features and aspects ofthe innovative technology disclosed herein.

What is claimed is:
 1. A apparatus, comprising: a substantially planartarget surface defining a plane; a ruled target surface inclined atsubstantially 45° to the substantially planar target and extendingthrough the plane thereof, marked to indicate respective distances infront of and behind the plane; and a fixture for holding thesubstantially planar target surface and the ruled target surface in adefined orientation to a camera, whereby a reflection of thesubstantially planar target surface and the ruled target surface is madevisible through the camera and its lens pointing at a mirror along aline of sight, in which reflection the substantially planar targetsurface and the ruled target surface are centrally disposed in a fieldof view along the line of sight and the substantially planar targetsurface is substantially perpendicular to the line of sight.
 2. Theapparatus of claim 1, further comprising a removable fastener forfastening the fixture to the camera.
 3. The apparatus of claim 1,wherein the fixture is configured to mount interposed between a tripodand the camera, and is configured for mounting the camera to the tripod.4. The apparatus of claim 3, further comprising the tripod attached tothe fixture.
 5. The apparatus of claim 1, wherein the fixture isconfigured to attach to a hot shoe for the camera.
 6. The apparatus ofclaim 1, wherein the fixture is configured to attach to the lens.
 7. Theapparatus of claim 1, wherein the fixture is configured to hold thesubstantially planar target surface so that the plane thereof isperpendicular to a reflected leg of the line of sight between the mirrorand the substantially planar target.
 8. The apparatus of claim 7,wherein the fixture is configured to hold the substantially planartarget surface so that a non-reflected leg of the line of sight betweenthe lens and the mirror is not parallel to the second portion of theline of sight.
 9. The apparatus of claim 1, wherein the substantiallyplanar target surface defining a plane and the ruled target surface areformed in a molded polymer material.
 10. The apparatus of claim 1,wherein the fixture is formed in a molded polymer material.
 11. A methodcomprising: using a camera, capturing a reflection of a focus pointmeasuring device, wherein the focus point measuring device is affixed tothe camera; and evaluating a captured image of the reflection to measurea calibration amount for a focus point, wherein the focus pointmeasuring device comprises: a substantially planar target surfacedefining a plane; a ruled target surface inclined at substantially 45°to the substantially planar target and extending through the planethereof, marked to indicate respective distances in front of and behindthe plane; and a fixture for holding the substantially planar targetsurface and the ruled target surface in a defined orientation to thecamera, whereby a reflection of the substantially planar target surfaceand the ruled target surface is made visible through the camera and itslens pointing at a mirror along a line of sight, in which reflection thesubstantially planar target surface and the ruled target surface arecentrally disposed in a field of view along the line of sight and thesubstantially planar target surface is substantially perpendicular tothe line of sight.
 12. The method of claim 11, further comprisingadjusting a focus point of a lens of the camera by the calibrationamount.
 13. The method of claim 11, further comprising fastening thefixture to the camera using a removable fastener.
 14. The method ofclaim 11, further comprising interposing the fixture between a tripodand the camera for mounting the camera to the tripod.
 15. The method ofclaim 14, further comprising attaching the tripod to the fixture. 16.The method of claim 11, further comprising attaching the fixture to ahot shoe for the camera.
 17. The method of claim 11, further comprisingattaching the fixture to the lens.
 18. The method of claim 11, furthercomprising holding the substantially planar target surface so that theplane thereof is perpendicular to a reflected leg of the line of sightbetween the mirror and the substantially planar target.
 19. The methodof claim 11, further comprising holding the substantially planar targetsurface so that a non-reflected leg of the line of sight between thelens and the mirror is not parallel to the second portion of the line ofsight.